Raster input scanner (RIS) systems have become a scanner of choice for electrophotographic and desktop printers. Typically, the RIS comprises a linear array of photosensitive elements or sensors mounted in optical alignment with a lens or a linear lens array. A document is illuminated by a linear light source, and a line-by-line scanning function is accomplished by the RIS with the image being detected and stored electronically. Outputs from the photosensitive array may be sent to a raster output scanner (ROS) which forms a modulated output pattern on a charged photoreceptor device or belt. Alternatively, the output may be used to drive other image-forming devices such as ink jet printers and the like, or may be sent to a remote location to be stored, for example, in a data processor.
Skewed images in RIS systems such as, for example, the Xerox DocuTech Production Publisher or the Xerox 7650 Scanner, are a common problem. While the skew of a copied image is always undesirable, in electronic imaging additional image quality defects occur, particularly jaggies or stair steps, noted whenever a line is drawn or skewed so as to be at angles other than 0.degree. or 90.degree.. In electronic images, skew arises in cases where 1) the information on the original document is skewed with respect to a document edge; 2) trapezoidal or other non orthogonally cut sheets; 3) document handler skew; and 4) anamorphic skew caused by non-orthogonal slow or fast scan imaging.
There are workaround methods of solving certain skew problems, although none are desirable. For class 1) and 2) skew problems, typical methods of correcting skew are to reposition an original document on a scanning platen, and re-acquire the image. Commonly, a platen cover or document handler is opened, the document is manually repositioned on the platen, and rescanned. Obtaining proper document positioning can be a slow and laborious trial and error method, requiring a proof copy after each adjustment. It is also a common experience that the movement of air caused by the closure of the platen cover causes document movement. In U.S. Pat. No. 4,916,483, small angle document rotation (skew angles are usually small) is accomplished by arranging a rotating document hold down arrangement, which frictionally engages a document in controlled manual rotation, whereupon the image may be rescanned at the correct angle. Unfortunately, the linkage allowing mechanical rotation does not lend itself to use in an automatic document handler. Additionally, the trial and error rescanning process is still required, as the rotator does not provide positional feedback. Class 3) problems cannot be resolved in this manner, since to do so would require interrupting the document handler. The class 4) problem is unique among the deskew causes, since it is a function of the image acquiring device, as opposed to the orientation of the document.
Certain image processing methods exist which provide small angle rotation (which is characteristic of document skew), as described for example in "A Fast Algorithm for General Raster Rotation", Graphics Interface '86, A. Paeth, p. 77 ff. (1986). However, such methods require some alteration of the image, and may cause image defects. Most image processing methods for rotation are memory intensive, and would slow copy production.
All of the references cited herein are incorporated by reference for their teachings.